Maroon-green-indigo color switching of thienoisoindigo-based electrochromic copolymers with high optical contrast

Background: Donor-acceptor (D-A) type polymers as electrochromic materials possess advantages in color regulation and performance optimization. However, the relationship between D-A type molecule structure and electrochromic properties is not very clear. Methods: Three D-A type copolymers, namely as DC-TIIG-X (X = 1, 2, 3), based on thienoisoindigo (TIIG) as acceptor unit, cyclopenta[2,1-b:3,4-b']dithiophene (DT) and carbazole (CZ) derivatives as donor units, were synthesized via Suzuki coupling reaction. The optical absorption properties of the copolymers could be regulated by changing the molar ratio of donor to acceptor. Significant findings: All the three solution-processed copolymers show low optical band gaps of 1.50-1.53 eV, ambipolar property, and good electrochromic characteristics. During p-doping and n-doping processes, neutral DC-TIIG-X (X = 1, 2, 3) could turn from green to indigo and maroon, respectively. Especially, DC-TIIG-2 exhibits large optical contrasts of 40%, 11% and 82% and high coloration efficiencies of 284.0 cm2 C 1, 78.9 cm2 C 1 and 433.4 cm2 C 1 at the wavelengths of 440 nm, 730 nm and 1600 nm, respectively. The optical contrast of 40% in visible region is one of the highest values in neutral green electrochromic polymers. Optimizing donor/acceptor molar ratio offers an effective strategy to design high-performance D-A type polymers as electrochromic materials.